History of Light Emitting Diodes (LED)
The earliest records of scientific observation of the luminescence of forward-biased diodes began in 1907 with the discovery of the LED. This invention was widely credited to the Russian technologist Oleg Vladimirovich Losev, who observed that zinc oxide and silicon carbide diodes used in the receiver circuits of radio sets he was working with emitted a small amount of light when forward biased. Losev's research on the cause and nature of diode luminescence became the foundation for a series of 16 papers published from 1924 to 1930 which described the results of his research on light-emitting diodes (Light Emitting Diodes or LEDs). Tragically, Losev was trapped in Leningrad during Hitler's Siege of Leningrad and died of starvation in 1942. With his untimely death and the confusion in the aftermath of WWII, his work fell into obscurity.
The history of the LED resurfaces again in 1962 when American researchers demonstrated a functioning LED-based laser. Between 1962 and the mid-1990s, the LED became a staple electronic component and was used in a variety of signage and signaling applications. The next major breakthrough in LED occurred in 1995 when Isamu Akasaki and H. Amano demonstrated an operational GaN-based High Brightness Light Emitting Diode (HBLED). The HBLED is much brighter, more efficient, and can be constructed to generate more colors than the traditional LED.
The electrical efficiency and compact size of the HBLED has opened up a vast array of new applications for these solid state lighting devices. In recent years, HBLEDs have found their way into a variety of scientific and illumination applications, fueling the expansion of the market for HBLEDs to over 11 billion dollars in the year 2006.
While the name “Light Emitting Diode” accurately describes the device from the perspective of a physicist, the name does not accurately describe how the device is used. From an application perspective, the HBLED is generally not used as a diode. A diode is used to pass current flow when forward-biased and to block current flow when reverse-biased. If the water analogy is invoked, the diode acts as a one-way flow valve. The primary function of an HBLED is to emit light. The HBLED converts electrical power to optical power. In the water analogy model, the diode is a one-way flow valve and the HBLED is a generator.
LED Testing
The test methodologies used to evaluate HBLED devices in the manufacturing process are based on traditional diode tests, broadened to include the measurement of the power and color (i.e. wavelength) of the HBLEDs optical output. The typical HBLED test starts with a simplified diode test sequence in which the diode is reverse-biased to determine the amount of leakage current and then forward-biased using a forcing current while the forward voltage is measured. To measure the optical properties of the HBLED, a forcing current is applied while a photodetector and/or spectrometer are/is used to measure the power and color of the light emitted by the HBLED Device Under Test (DUT).
Testing HBLEDs using the traditional test methodology in the production environment is especially challenging as these tests must be performed in the shortest amount of time possible—often 50 milliseconds or less. In most cases, the production HBLED test sequence is completed before the device being tested has reached thermal equilibrium, which significantly reduces the accuracy and repeatability of the forward voltage measurement as well as the optical power and color measurements.
From a test and measurement perspective, since the HBLED is fundamentally an energy conversion device, one would expect the test methodology or strategy used to evaluate a HBLED would be very different than a diode test methodology or strategy.